The accumulation of a NADPH-specific glutamate dehydrogenase is inducible, even during periods of nuclear chromosome condensation, by addition of ammonium to previously uninduced synchronous cells of the eucaryotic microorganism, Chlorella, at any time during its cell cycle. The initial rate of induced enzyme accumulation increases early in S-phase in proportion to the increase in gene dosage. In synchronous cells growing in the continuous presence of inducer, however, the expression of newly replicated genes of this enzyme is delayed until long after S-phage. The removal of inducer (deinduction) results in the very rapid decay (t 1/2 equals 10 min) in induced enzyme activity. The objectives of the research are to characterize the molecular mechanisms regulating or rate-limiting (a) the increase and decrease in induced enzyme activity during induction and deinduction, respectively, and (b) the delay between gene replication and gene expression in synchronous cells cultured in continuous presence of inducer. Specific immunological and biochemical procedures will be used in measurements of the rates of induced enzyme synthesis and degradation in in vivo labeling studies, in the quantitation of total translatable mRNA of the enzyme in vitro heterologous translation systems, and in an indirect immunoprecipitation procedure for isolation and purification of the specific mRNA. After mRNA purification, cDNA will be made from it and used in hybridization studies to quantify possible nontranslatable mRNA sequences of the enzyme in different cellular fractions during the cell cycle.